The present invention generally relates to a limit switch assembly and, more particularly, to a limit switch assembly of a type comprising a switch box having a built-in switch, and a head mounted on the switch box and having an actuator for actuating the switch.
Hitherto, numerous types of limit switch assemblies have been developed and placed in the market, some of them employing the actuator in the form of a plunger and some of them employing the actuator in the form of a lever or a combined lever and arm. In all of these prior art assemblies, not only are the head and the switch box connected together with an O-ring interposed therebetween, but the built-in switch is secured in position within the switch box by the use of set screws. Accordingly, it has been found that, even though the O-ring is interposed between the head and the switch box, the O-ring is not effective to avoid any possible intrusion of an undesirable fluid medium such as, for example, oil by the capillary action, and therefore, the built-in switch tends to fail to operate properly when wetted in contact with the fluid medium. In addition, according to the prior art, since the area of the switch from which a push button projects outward is exposed to the outside of the switch box before the head is mounted on the switch box with the actuator aligned with the push button, foreign matter such as dusts and oily substances tend to adhere to the push button which may ultimately result in the malfunctioning of the switch assembly as a whole.
The intrusion of oil into the built-in switch by the capillary action may also take place through the set screws used to secure the switch in position within the switch box, thereby posing a problem similar to that described above. Furthermore, where the switch is secured in position within the switch box by the use of the set screws, impacts and/or vibrations applied exteriorly to the switch box tend to be transmitted to the built-in switch, and, in the worst it may happen, not only does the built-in switch fail to operate properly, but also external wiring elements connected to respective terminal members of the built-in switch may separate from the terminal members.
In view of the foregoing, it can be contemplated to use a rubber hood in a manner as shown in FIG. 9 of the accompanying drawings. Referring to FIG. 9, the rubber hood, generally identified by 34, is configured to substantially tightly receive therein the switch 33 contacting all of the surfaces of said switch including the surface thereof from which the push button protrudes outwards, but excluding the surface thereof from which terminal members protrude outwards. The switch 33 so covered by the rubber hood 34 is secured to a wall of the switch box 20 by means of screws 60 extending through respective mounting holes 33c defined in the switch 33 so as to extend completely through the thickness thereof.
In the contemplated arrangement shown in FIG. 9, both the impact resistance and the dust proofness of the switch assembly may be improved. However, a worker engaged in the fabrication of the switch assembly will be forced to locate the mounting holes 33c exteriorly of the rubber hood 34 immediately before the screws 61 are inserted therethrough to secure the switch to the wall of the switch box 20. Even if the switch will have been successfully secured to the switch box 20, portions of the rubber hood adjacent the screws extending through the respective mounting holes may be inwardly compressed, as shown, as the screws are fastened, and accordingly, in the assembled condition, the switch 33 may happen to be secured in a manner tilted relative to the wall of the switch box 20. Once this happens, the operating position of the actuator for actuating the push button of the switch may deviate from a predetermined or required position to such an extent as to fail to properly engage an external driving element.